class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> lists = new ArrayList<String>();
        if (digits.length() == 0) {
            return lists;
        }
        Map<Character, String> phoneMap = new HashMap<Character, String>() {{
            put('2', "abc");
            put('3', "def");
            put('4', "ghi");
            put('5', "jkl");
            put('6', "mno");
            put('7', "pqrs");
            put('8', "tuv");
            put('9', "wxyz");
        }};
        backtrack(lists, phoneMap, digits, 0, new StringBuffer());
        return lists;
    }

    public void backtrack(List<String> lists, Map<Character, String> phoneMap, String digits, int index, StringBuffer strbuf) {
        if (index == digits.length()) {
            lists.add(strbuf.toString());
        } else {
            char digit = digits.charAt(index);
            String letters = phoneMap.get(digit);
            int lettersCount = letters.length();
            for (int i = 0; i < lettersCount; i++) {
                strbuf.append(letters.charAt(i));
                backtrack(lists, phoneMap, digits, index + 1, strbuf);
                strbuf.deleteCharAt(index);
            }
        }
    }
}